Tooth movement derived by PDL Cellular Manipulations The ability of teeth to move is essential for their adaptation to the constantly changing environment and therefore for their survival. The periodontal ligament (PDL) has a central role in controlling this tooth movement. However, despite the fundamental role of the PDL, the functional mechanisms and the 3D structure of this remarkable tissue are not fully understood. The proposed study is aimed at elucidating the structure-function basis of the PDL and by doing so to generate a new clinical method for controlled tooth movement. Loads change the fibrous distribution inside the PDL in order to enable certain functions. For instance, orthodontic loads cause changes in PDL fiber structure and trigger a cascade of bone remodeling that eventually leads to a change in the position of the tooth. It was recently shown that the PDL is not uniform in its structure and function even under normal functional loads; in other words, tooth function controls the structure of the PDL. The hypothesis of this proposed project is that intentional modifications in the PDL structure will trigger changes in the surrounding bone and eventually functional tooth changes. This hypothesis will be addressed in four complementary specific aims. The mentored phase (K99) will be conducted with Dr. Bjorn Olsen as mentor and Dr. Leslie Will as a co-mentor, at Harvard School of Dental Medicine and at Boston University School of Dental Medicine. The experiments in this phase are anticipated to establish a 3D characterization of the collagen networks and the cellular populations of the mouse molar tooth PDL. To do so, we will utilize an arsenal of 3D imaging methods, we will use a unique custom-made apparatus inside a microCT as well as 2 photon and 2nd harmonic-generation imaging. For high resolution structural analysis we will also employ STORM and AirSEM methods. We will cross different genetically modified mouse strains to investigate the cellular populations in the PDL and we will take advantage of in-situ hybridization to investigate gene expression levels as well as signaling pathways. Crossing disciplines, bioengineering tools will be used for in- vitro investigation of the PDL stiffness and how to alter it. The R00 phase wil establish a deep understanding of the association between structure and function of the PDL and will develop a mechanism to control PDL function by structural modifications. This project includes a well-structured training program that provides theoretical course work, practical experience and protected research time during individualized specialty training in Orthodontics. The final result of this project, controlling tooth movement through modifications of cellular activities inside the PDL, will be a novel approach with the potential of greatly advancing the discipline of orthodontics and benefiting other medical disciplines as well.